Late Spring Frost Damages Pecan Trees

     A strong cold front pushed its way across Kansas on Tuesday April 20. We even saw light snow showers during the daylight hours. But as nightfall approached the sky cleared off and temperatures started to drop. During the early morning hours of Wednesday temperatures hovered around 28 degree F (-2 C) for at least 4 hours. Emerging pecan shoots can withstand light sub-freezing temperatures but 28 degrees seems to be the critical temperature for damage to begin. I took the photo you see above on the afternoon of April 21st.  Two adjacent Kanza shoots can be seen, one damaged by the cold (shoot on left) and one perfectly healthy (shoot on right). This one photo typifies the kind of damage I have seen following this cold weather event. The amount of damage is quite variable and I have no easy explanation for why some shoots were injured while other not. 

    Several new shoots appeared burned by the frost as pictured at left. I've seen this type of frost damage following spring freezes in years past. The outer leaves of the emerging shoot are damaged and will never fully expand to form a normal functional leaf. However, the health of the shoot's growing point is what actually determines if this frost damaged terminal will be able to set a pistillate flower cluster.

   I pulled back the outer leaves of the new shoot and found healthy green tissue at the growing point (photo at right).  What looks like terrible damage today will most likely not impact the yield potential of the shoot.

    For young and recently transplanted pecan trees the amount of freeze damage seems consistently worst. Temperatures are typically colder closer to the ground during cold, clear nights. If you were planning of grafting any young stock this spring, I recommend waiting until you see new green shoots develop on rootstock trees before sharpening your grafting knife.
   

New pecan shoots emerging

     With a week of cool days and even cooler nights, pecan shoot growth has slowed to a crawl. However, when I looked at my trees a few days ago I could already see new leaves and catkins emerging from terminal branches. The photo at right shows the terminal of a 'Gardner' branch. The stem seems to be covered with emerging catkins with a single vegetative bud at the top of the shoot. This type of spring bud emergence is typical for a protandrous pecan cultivar such as 'Gardner'. The word, "protandrous", can be roughly translated as "first male" and describes a pecan cultivar that sheds pollen before the pistillate flowers on the same tree become receptive to pollination.  

    The photo at left shows the terminal of a 'Kanza' shoot. Vegetative growth on this shoot is far more advanced with catkins only now starting to emerge from their bud scales. 'Kanza' is a protogynous cultivar. The word "protogynous" can be loosely translated as "first female". Pecan pistillate flowers are produced on the ends of new shoots which means that protogynous cultivars prioritize shoot growth over catkin growth during the initial phases of bud development. Ultimately, this advanced vegetative growth will lead to the pistillate flower clusters becoming receptive during the earliest part of the pollination season. 

   In the grand scheme of things, a protandrous cultivar will pollinate a protogynous cultivar and vice versa. Since pecan trees are wind pollinated this dicogamy of flowering types ensures that nuts form as the result of cross pollination. Cross polination imparts hybrid vigor in the next generation of pecan trees (seedling trees grown from nuts) ensuring the continued survival of the species.

Pecan trees breaking bud

     Over the past week, I've been enjoying the spring weather by working in the pecan grove. Today, I collected terminal branches from several trees to illustrate the variation that occurs in time of budbreak between pecan cultivars. The photo below shows the stage of bud development for six pecan cultivars.  

 

    Greenriver buds were the most advanced with new leaves primed to unfurl. In sharp contrast, Hark buds were just starting to swell with their out scales recently split open.

   When folks look at variation among pecan cultivars they often try to make generalizations in terms of northern pecan cultivars vs. southern cultivars.  However, the earliest bud breaking cultivars shown above (Greenriver and Yates 68) are widely planted northern pecans. Lakota and Kanza are USDA creations that contain genes from both a southern cultivar and Major (a northern cultivar). Gardner and Hark are open pollinated seedlings of unknown parentage. From my experience, I have noticed an equal amount of variation in bud development among the native pecan trees that populate the Neosho River bottoms.   

    The timing of bud development has one important ramification. Trees that break bud early are more often damaged by late spring frosts. Over the years, I can clearly remember Greenriver buds suffering frost damage.   However, with cold enough temperatures, all stages of green tissue on pecan trees can be injured.